The present invention relates to a differential amplifier with offset voltage trimming and a method for offset voltage trimming. The invention is particularly useful for automatic offset voltage trimming of a differential amplifier that supplies an output voltage to control a voltage dependent load such as a Voltage Controlled Oscillator (VCO).
Differential amplifiers, typically operational amplifiers, often have an offset voltage at the amplifier's output caused by component value imbalances primarily resulting from temperature drift. Such offset voltages are undesirable especially when highly accurate output voltages are required, for instance, when used for controlling a VCO. One solution for trimming (reducing) the offset voltage involves canceling the offset voltage with another voltage of the same magnitude and opposite polarity. This solution, in its simplest form, involves inserting a variable resistance into the inverting input path of the amplifier. However, the exact magnitude and polarity of the offset voltage may vary due to temperature drift and thus the variable resistance may need to be adjusted frequently. Further, the insertion of the variable resistance into the inverting input path can affect the gain of the amplifier.
Another solution for trimming offset voltages includes selectively inserting resistors into the paths of differential transistor pairs that are part of the amplifier's internal circuitry. However, this solution also affects the gain of the amplifier, which may not be suitable for specific applications.
Accordingly, it would be advantageous to have a differential amplifier that produces a highly accurate output voltage, and to be able to trim an offset voltage without adversely affecting amplifier gain.